(1) Field of the Invention
The present invention relates to the field of airspeed indicator installations for rotorcraft.
Aircraft are conventionally provided with an airspeed indicator installation that supplies the pilot with information about the airspeed of the aircraft, i.e. the speed at which said aircraft is moving relative to the air in which it is flying. Such an installation conventionally makes use of at least one airspeed sensor measuring the characteristics of the speed of the air stream surrounding the aircraft, and more particularly its speed vector components extending respectively substantially along at least the longitudinal axis and/or the transverse axis of the aircraft.
(2) Description of Related Art
In this context, the airspeed indicator installation provides the pilot of the rotorcraft, in particular by means of a display, with the speed of the aircraft relative to said surrounding air stream, referred to as the airspeed, on the basis of measurements supplied by the airspeed sensor and defined in terms at least of a longitudinal airspeed component, and possibly also of a lateral airspeed component.
The ground speed of the aircraft is conventionally supplied by an on-board instrument of the aircraft, typically such as a satellite location appliance, for example. Under such conditions, knowledge of the airspeed of the aircraft makes it possible to identify the characteristics of the wind to which the aircraft is subjected.
The airspeed sensors that are conventionally used in aviation commonly make use of at least one Pitot tube. Such airspeed sensors enable the airspeed of the aircraft to be deduced from measurements supplied by the Pitot tube(s) for the static air pressure and for the total air pressure of the air surrounding the airspeed sensor.
Nevertheless, the main source of error in an airspeed sensor lies in it being impossible for the airspeed sensor to be continuously in alignment with the air stream surrounding the aircraft. Certain airspeed sensors cannot align themselves with this air stream.
In this context, omnidirectional airspeed sensors have been developed that provide airspeed measurements defined using vector components, and in particular at least a longitudinal component and a lateral component.
Conventionally, the vector components of the airspeed as measured by an omnidirectional airspeed sensor are identified along the general axes in which the aircraft in question extends when on the ground. The longitudinal component of the airspeed is commonly defined along the longitudinally extending axis of the aircraft that extends between the front and the rear of the aircraft. The lateral component of the airspeed is commonly defined along the transversely extending axis of the aircraft that extends between the right and left sides of the aircraft.
In aviation, use is made more particularly of omnidirectional airspeed sensors of the low range airspeed system (LORAS) type. LORAS type airspeed sensors make use of a pair of Pitot tubes that are rotatably mounted and arranged in opposite directions so that such LORAS airspeed sensors can measure the longitudinal component and the lateral component of the airspeed.
To measure the airspeed of an aircraft, it is also known to make use of remote-detection omnidirectional airspeed sensors that make use of light rays of the laser beam type or of soundwaves of the ultrasound type.
By way of example, optical airspeed sensors, such as light detection and ranging (LIDAR) airspeed sensors enable the airspeed of an aircraft to be measured by sequentially transmitting and receiving a laser light beam at a given repetition rate.
Also by way of example, ultrasound airspeed sensors, such as the airspeed sensor described in Document U.S. Pat. No. 4,031,756 (Honeywell) for example, makes it possible to measure the airspeed of an aircraft by transmitting and receiving ultrasound waves.
In this technological context relating to airspeed indicator installations for aircraft, it is necessary among powered aircraft to take into consideration the features that are specific to rotorcraft.
Rotorcraft are aircraft with one or more rotary wings in which at least lift is provided by at least one main rotor having a rotary drive axis that is substantially vertical. In the specific context of a helicopter, the main rotor provides the rotorcraft not only with lift, but also with propulsion in any travel direction.
Aircraft are also commonly fitted with at least one auxiliary rotor having a rotary drive axis that is substantially horizontal. Such an auxiliary rotor constitutes in particular an anti-torque device serving to stabilize and guide the rotorcraft in yaw. By way of example, such an auxiliary rotor is a tail rotor installed at the end of a tail boom of the rotorcraft, and sometimes surrounded by a fairing that, among other advantages, serves to reduce the sound nuisance generated by the tail rotor.
Rotorcraft are also commonly provided with stabilizers serving to stabilize the flight attitude of the rotorcraft, or indeed to modify it.
By way of example, such stabilizers may be formed by wings extending generally in a substantially horizontal plane of the rotorcraft as defined by the longitudinal and transverse axes of the rotorcraft, considered when standing on the ground. Such stabilizers may also be formed by fins that extend generally in a substantially vertical plane of the rotorcraft, considered when standing on the ground, said vertical plane being oriented perpendicularly to said horizontal plane.
Such fins are conventionally placed at the rear of the rotorcraft, in particular at the end of a tail boom, and they are commonly inclined relative to the vertical plane of the rotorcraft.
Compared with other aircraft, measuring the airspeed of a rotorcraft raises specific problems that result in particular from the presence of rotors that, by rotating, disturb the air stream around the rotorcraft. In this context, there arises the problem of where to place the airspeed sensor on board on the rotorcraft in order to obtain the most pertinent possible information about the airspeed of the rotorcraft.
Specifically, a rotor produces a wake as a result of its own rotation. This means that the pertinence of the measurements supplied by the airspeed sensors in order to obtain information about the airspeed of the rotorcraft depends on locating the airspeed sensor on the rotorcraft while protecting it as well as possible from the aerodynamic effects generated by the rotor(s).
Under such conditions, and in particular concerning Pitot tube airspeed sensors, the location for the airspeed sensor on board a rotorcraft may for example be selected to be above the main rotor and on its rotary drive axis, as described in Document US 2006/0027702, or indeed at the front of the rotorcraft, as described in particular in Documents WO 01/74663 and EP 1 462 806.
Concerning remote-detection airspeed sensors, their locations on board a rotorcraft are conventionally selected to be at the front of the rotorcraft and more specifically in the nose of the rotorcraft, with the airspeed sensor facing in the forward travel direction of the rotorcraft.
There is also the problem of measuring the airspeed of a rotorcraft over the entire flight envelope of the rotorcraft.
Compared with other powered aircraft, rotorcraft present the particular feature of being capable of hovering and/or of flying at low speeds, which are typically considered as being speeds lower than 50 knots (kt). Nevertheless, at speeds lower than 50 kt, simple Pitot tube airspeed sensors, regardless of how high their performance might be, do not enable reliable measurements to be obtained of the airspeed of the aircraft. Specifically, the accuracy of a Pitot tube decreases with decreasing airspeed.
It is common practice to warn the pilot that information about the airspeed of a rotorcraft is not available while flying at low speeds. More particularly, while the rotorcraft is flying at speeds that are higher than low speeds, the airspeed of the rotorcraft is calculated on the basis of measurements provided by the airspeed sensor and it is communicated to the pilot by means of a display. At low travel speeds of the rotorcraft, it is then common practice to display information to the effect that the airspeed of the rotorcraft is not available.
That is why there is a continuous search in the field of rotorcraft to provide a pilot with information that is as reliable as possible about the airspeed of a rotorcraft flying at low speeds and/or while hovering. For this purpose, calculation systems have been developed that enable the airspeed of a rotorcraft to be determined from the flight commands issued by the pilot.
For example, the airspeed of the rotorcraft may be deduced by taking account of the overall flying attitude of the rotorcraft as identified by the current status of its flight mechanism, e.g. by detecting variation in the pitch of the blades of the rotor(s), and in particular of the main rotor. By way of example, reference may be made on this topic to the following Documents: FR 2 567 270 (Durand) and FR 2 648 233 (Crouzet).
Nevertheless, such calculation systems are complex and expensive. Since the measurements supplied by simple Pitot tube airspeed sensors are reliable at high travel speeds of the rotorcraft, it is found in practice that, while the rotorcraft is flying at low speed, a simple Pitot tube airspeed sensor continues to be used for measuring the airspeed of a rotorcraft to the detriment of obtaining airspeed information, with this applying in particular for rotorcraft that are commonly referred to as being “light”, and that need in particular to be as inexpensive as possible.
It consequently appears appropriate to continue the search seeking to obtain the most reliable possible measurement of the airspeed of a rotorcraft flying at low speeds by means of an airspeed sensor.
For this purpose, the conventional approach of designers is to improve and/or design airspeed sensors that are suitable for providing the pilot with a measurement of the airspeed of a rotorcraft when it is flying at speeds that are particularly low. In order to obtain measurements that are as reliable as possible at low flying speeds of a rotorcraft, airspeed sensors are conventionally adjusted as from their initial design so as to operate in given measurement ranges.